This invention relates to a method of controlling the air-fuel ratio in internal combustion engines, and in particular, to a method which makes it possible to adjust the air-fuel ratio close to a theoretical air-fuel ratio with accuracy even under a transient state such as acceleration and deceleration.
A method of adjusting the air-fuel ratio in internal combustion engines as close as possible to a theoretical air-fuel ratio has been proposed in U.S. Pat. No. 4,616,619. According to this method, the fluctuation of a signal supplied from an air-fuel-ratio sensor when the engine is being accelerated is monitored to measure the deviation of the actual air-fuel ratio from a theoretical ratio, and an acceleration fuel-increment coefficient or a deceleration fuel-decrement coefficient is learned in such a manner that this deviation becomes zero.
However, the above method has the following problem: depending upon the type of factor causing variation in the transient air-fuel ratio, it may sometimes be difficult for a transient air-fuel ratio to be adjusted to the theoretical ratio over the entire engine-warm-up range. According to the result of an experiment conducted by the inventors of the present invention, the manner of variation in a transient air-fuel ratio under different engine-temperature conditions (e.g., different engine coolant temperatures) greatly varies depending upon the type of factor causing the variation (which may, for example, be deposit around the intake valve or the properties of the gasoline used).
In the case where valve deposit constitutes the factor, the air-fuel ratio varies to a large degree as the temperature of the coolant changes. In the case where the gasoline properties constitute the factor, the air-fuel ratio does not vary so much with the temperature of the coolant. This fact indicates that the degree of dependence of the variation in air-fuel ratio upon the temperature of the coolant is completely different for different factors causing the variation.
Thus, with the above-described conventional method, which does provide for discrimination of one type of factor from the other, the air-fuel ratio cannot be adjusted to the theoretical ratio over the entire temperature range of the coolant.
This problem may be solved by establishing different learning values for different temperature ranges of the coolant. With such a system, however, the learning cannot be conducted satisfactorily on the lower-temperature side, so that a problem arises with respect to the learning speed. That is, since the temperature of the coolant is raised too soon during the engine warm-up period, there is scarcely any chance for the learning to be conducted on the lower-temperature side. Thus, the above problem cannot be solved by simply establishing different learning values for different temperature ranges of the coolant, since the learning is not then effected satisfactorily on the lower-temperature side, resulting in an excessive deviation from the theoretical air-fuel ratio.